From the related art, e.g., the Automotive Handbook of Robert Bosch GmbH, 26th edition, January 2007, pages 810 and 811, tire-pressure control systems (tire pressure monitoring system TPMS) are discussed for monitoring the tire pressure in vehicles; these systems are meant to prevent tire defects resulting from insufficient tire pressure and thereby reduce the number of accidents attributable to defective tires.
Operating a vehicle at insufficient tire pressure will result in greater flexing work along the sides of the tire and thus in increased wear of the tires. At high accelerations a tire weakened in this manner may possibly no longer be able to withstand the stressing and burst. Following an increase in serious accidents with fatalities in the U.S. due to burst tires resulting from reduced pressure, a law has been passed (NHTSA Tead Act), which regulates the introduction of tire-pressure control systems across the entire U.S.A. in order to provide drivers with an early warning in the future if reduced tire pressure is detected. As of September, 2007, all new vehicles must be equipped with tire-pressure control systems. In addition to tire damage, “creeping flats” due to the gas diffusion through the tire rubber over time are a major cause of decreased tire pressure.
However, the tire inflation pressure is not only an important variable for traffic safety. The inflation pressure clearly affects driving comfort, tire service life and fuel consumption as well. A decrease of 0.6 bar in the inflation pressure may increase the fuel consumption by up to 4% and shorten the service life of the tire by up to 50%.
The increasing percentage of tires having emergency running properties likewise necessitates the use of tire-pressure control systems because the driver of a vehicle is no longer able to detect a tire having considerably reduced pressure (flat tire) based on the driving behavior. To prevent drivers from unknowingly exceeding the speed and distance limits that apply in such a case, emergency-running tires may only be used in conjunction with tire-pressure control systems or flat-rolling warners.
In direct tire-pressure control systems, a separate sensor module including a pressure sensor is installed in each tire of the vehicle. Via a coded high-frequency transmission link, data from the tire interior, such as tire pressure and tire air temperature, is transmitted by this module to a control unit. These data may be analyzed in the control unit, thereby detecting not only pressure losses in individual tires but also slow pressure losses in all tires (for instance due to diffusion through the rubber). If the tire pressure drops below a specified threshold or if the pressure gradient exceeds a specific value, then the driver is warned by a visual or acoustic signal.
The sensor module is normally fixed in place in the tire together with the tire valve. The modules are usually supplied by a battery. This results in additional demands with regard to power consumption, media resistance and acceleration sensitivity in comparison with other applications. Micromechanical absolute-pressure sensors are used as sensor element.
The data measured inside the tire with the aid of pressure and temperature sensors are processed in the sensor module, modulated upon an HF carrier signal (433 MHz in Europe, 315 MHz in the U.S.), and emitted via an antenna. This signal is detected either via individual antennas on the wheel wells or in a central receiver (e.g., in the control unit of existing remote keyless entry systems).
Furthermore, in commercial vehicles conventional brake systems, activated and controlled purely via a pressure arrangement, are supplemented by anti-lock braking systems (ABS) or have been replaced by electronically controlled brake systems (EBS) having integrated ABS. Both systems are controlled brake systems; in an anti-lock braking system (ABS) an ABS control unit adjusts the slip of braked wheels as a function of a deviation between the actual slip and a setpoint slip, either singly or per axle, and in an electronically controlled brake system (EBS), an EBS brake control unit adjusts the brake pressure in brake cylinders of braked wheels as a function of a deviation between the actual brake pressure and a setpoint brake pressure.
At present, the sectors of the controlled brake systems—EBS having integrated ABS or ABS alone—for one, and the tire-pressure control systems or the tire-pressure regulation systems, for another, are separate from one another.